This invention relates to a magnetic head for use in a magnetic recording and reproducing apparatus, more specifically a magnetic head having excellent mass productivity, wear resistance, mar resistance, weather resistance and magnetic properties, bonding glass used therefor and a magnetic recording and reproducing apparatus adapted with said magnetic head.
In the conventional magnetic heads, there have been used single body of ferrite as magnetic core, principally SiO.sub.2 as non-magnetic gap material and SiO.sub.2 -B.sub.2 O.sub.3 based or SiO.sub.2 -PbO based glass having a bonding temperature in the range of 700.degree.-800.degree. C. for bonding of the magnetic cores.
Recently, for achieving higher recording performance (recording at higher density) of the magnetic recording and reproducing apparatus such as VTR, the further improvements of recording art such as higher coercive force of the information recording medium, wider bands of recording and reproducing frequencies, and higher speed of recording tape or disc have come to be required, and it has become difficult for the conventional magnetic heads to meet these requirements. Under these circumstances, attention is directed to magnetic films of such materials as Co-based amorphous alloys, Sendust based alloys and Fe-C based materials which have higher saturation magnetic flux density than ferrite, for use as magnetic core, and some of such magnetic films have been commercialized. The saturation magnetic flux density of these magnetic films is over 8,000 G, which is far higher than that (4,000-5,000 G) of ferrite, so that use of a magnetic head incorporated with such a magnetic film serving as magnetic core may enable recording at far higher density than possible with the conventional magnetic heads. Said magnetic films, however, are inferior to ferrite in temperature resistance, so that glass having a bonding temperature of 700.degree.-800.degree. C. employed for the conventional magnetic heads is unappliable to the magnetic heads using said magnetic film. So, there is employed PbO-B.sub.2 O.sub.3 based glass which is capable of low-temperature glass bonding. As the non-magnetic gap material, SiO.sub.2 is used as heretofore in anticipation of a certain degree of reaction with this type of glass.
PbO-B.sub.2 O.sub.3 based glass, however, had the problem that the production yield of magnetic heads was low due to notably low chemical stability (low water and moisture resistance, etc.) and poor mechanical properties such as hardness and strength in comparison with the glass used for the conventional magnetic heads. Said glass also involved the problem that when a magnetic tape is let run around the magnetic head, the glass bonded part thereof was apt to wear and also susceptible to scratch. These problems become more conspicuous when a magnetic recording tape or such formed by using deposition or sputtering techniques is let run or when it is let run in a high-humidity state. Said PbO-B.sub.2 O.sub.3 based glass had the further problem of corrosive action on the magnetic film at the time of glass bonding, making it hard to obtain a magnetic head having the desired magnetic properties. For solving these problems, there were proposed a glass composition and a magnetic head disclosed in Japanese Patent Application Kokai (Laid-Open) Nos. 62-78128 and 62-88109. These patent applications disclose V.sub.2 O.sub.5 -P.sub.2 O.sub.5 -Sb.sub.2 O.sub.3 based glass having better chemical stability and mechanical properties than the conventional PbO-B.sub.2 O.sub.3 based glass and a high-performance magnetic head improved in reliability by using said new glass. This bonding glass for magnetic head is defined in its composition as follows in consideration of appropriate coefficient of thermal expansion and bonding temperature, improvement of water resistance, prevention of crystallization and other matters: preferably V.sub.2 O.sub.5 =55-70 wt %, P.sub.2 O.sub.5 =17-25 wt %, Sb.sub.2 O.sub.3 =3-20 wt %, PbO=0-20 wt %, Tl.sub.2 O=0-15 wt % and Nb.sub.2 O.sub.5 =0-5 wt %, more preferably V.sub.2 O.sub.5 =55-65 wt %, P.sub.2 O.sub.5 =18-22 wt %, Sb.sub.2 O.sub.3 =5-12 wt %, PbO=3-10 wt %, Tl.sub.2 O=3-10 wt % and Nb.sub.2 O.sub.5 =0.5-2 wt %.
In the prior art described above, no consideration has been given to the cells formed at the time of glass bonding of the magnetic head and reactivity between bonding glass and non-magnetic gap material. Therefore, in the case of said conventional V.sub.2 O.sub.5 -P.sub.2 O.sub.5 -Sb.sub.2 O.sub.3 based glass within the defined range of preferred composition, a large number of cells would be generated at the time of glass bonding, and if such cells were present in the sliding face of the magnetic head, they would be filled up with fine powder of the recording medium when the recording medium such as recording tape is let run, thus deteriorating the magnetic head performance. Also, said V.sub.2 O.sub.5 -P.sub.2 O.sub.5 -Sb.sub.2 O.sub.3 based glass are poor in reactivity with SiO.sub.2 used as non-magnetic gap material. Therefore, in the case of magnetic head of the type in which bonding glass and non-magnetic gap material are reacted to bond together a pair of magnetic cores, firm and solid glass bond can hardly be expected as far as SiO.sub.2 is used as non-magnetic gap material.